---
title: Before your blood test
type: index
url: "https://www.test-well.com/prep"
review_status: reviewed
---

# Before your blood test: what can skew your results

> One page per factor — fasting, biotin, alcohol, exercise, thyroid medication timing, supplements, time of day, menstrual cycle day, illness, hydration, common medications — with which markers move, in which direction, for how long, and what to do about it.


- [Do you need to fast before a blood test — and which tests does eating affect?](https://www.test-well.com/prep/fasting) — Fast 8–12 hours (water is fine) for fasting glucose, fasting insulin and a lipid panel — eating raises glucose and triglycerides for hours.
- [Does biotin affect blood test results?](https://www.test-well.com/prep/biotin) — Yes, and seriously.
- [How does alcohol affect blood test results?](https://www.test-well.com/prep/alcohol) — A drinking session in the 24–48 hours before a draw can raise triglycerides, liver enzymes (especially GGT and AST), uric acid and glucose, and depress blood sugar hours later.
- [Should you exercise before a blood test?](https://www.test-well.com/prep/exercise) — Skip strenuous exercise for 24–48 hours before a draw.
- [Should you take levothyroxine before a thyroid blood test?](https://www.test-well.com/prep/levothyroxine-timing) — Draw before that day's dose when the order includes Free T4 or total T4: a levothyroxine tablet raises T4 in the blood for several hours afterward, while TSH barely moves within a day.
- [Should you stop iron supplements before an iron blood test?](https://www.test-well.com/prep/iron-supplements) — Hold your iron pill on the morning of the draw and test fasting: a supplement taken hours earlier can push serum iron and transferrin saturation well above your true baseline.
- [Should you take vitamin D before a vitamin D blood test?](https://www.test-well.com/prep/vitamin-d-supplements) — You don't need to stop.
- [Does the time of day change blood test results?](https://www.test-well.com/prep/time-of-day) — For several hormones, yes — a lot.
- [What day of your cycle should you get hormone blood tests?](https://www.test-well.com/prep/menstrual-cycle) — It depends on the hormone.
- [Should you postpone a blood test if you've been sick?](https://www.test-well.com/prep/recent-illness) — For most routine and screening tests, yes — wait about two weeks after a cold, flu, COVID or any infection.
- [Does dehydration affect blood test results?](https://www.test-well.com/prep/hydration) — Yes.
- [Which common medications affect blood test results?](https://www.test-well.com/prep/medications) — Several everyday medicines change results in predictable ways: statins can raise CK and liver enzymes; oral contraceptives raise SHBG and lower free testosterone; metformin lowers vitamin B12 over time; long-term acid reducers (PPIs) lower magnesium and B12; steroids raise glucose and white cells and suppress cortisol; and finasteride roughly halves PSA.

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